Abstract The nucleation and growth of CdS has been studied experimentally by allowing H 2S to react with a solution of cadmium salt. The precipitation was followed by simultaneously measuring H 2S pressure, conductivity and optical density of the solution, cadmium ion activity, and pH. Variables were [Cd 2+], rate of admission of H 2S, complexing ions present, and solvent. These variables determined particle size and structure (cubic or hexagonal). The complexes present appear to play a key rôle in determining whether Ostwald ripening takes place, and in the formation of secondary clumps of crystals called boules.